The present invention relates to a process for producing optically active cephalosporin analogs represented by the formula ##STR1## wherein R.sub.1 represents a substituted or unsubstituted saturated or unsaturated six-membered carbocyclic or five-membered heterocyclic group, R.sub.2 represents a hydrogen or a lower alkyl group, R.sub.3 represents a hydrogen or a protective group of carboxylic acid, the hydrogens at the 6- and 7-positions have cis configuration and X represents a hydrogen, a lower alkyl group, a hydroxy group, a carboxy group or an amino group and salts thereof.
A carbacephem compound, which is named according to the nomenclature in J. Am. Chem. Soc. 96, 7582 (1974), wherein the sulfur atom of cephalosporin is substituted with a carbon atom and which has a substituted methyl group at the 3-position is described in the above reference and J. Med. Chem. 20, 551 (1977).
Heretofore, the present inventors have succeeded in preparing carbacephem compounds having various substituents at the 4-, 5- and 3-positions. The compounds are described in the specifications of Japanese Patent Application Nos. 34696/78, (Japanese Published Unexamined Patent Application No. 128591/79), 122403/78, 133072/78, 162005/78 and 8408/79, U.S. patent application Ser. No. 23,645 (hereinafter "U.S. Ser. No. 23,645") filed Mar. 23, 1979 now U.S. Pat. No. 4,291,164 and German Offenlegungsschrift No. 2911786 (hereinafter "G.O. No. 2911786").
Furthermore, the present inventors have succeeded in preparing novel acylated carbacephems which are new antibiotics having strong antibacterial activity. The compounds are described in Japanese Patent Application Nos. 34696/78, 122402/78, 127027/78, 133071/78, 162006/78, 162007/78 and 8409/79, U.S. patent application Ser. No. 23,646 filed Mar. 23, 1979 (hereinafter "U.S. Ser. No. 23,646") and German Offenlegungsschrift No. 2911787 (hereinafter "G.O. No. 2911787").
As is described in the above-mentioned specifications, the cephalosporin analogs are prepared by synthetic methods using optically inactive starting compounds, and the final products are essentially optically inactive dl [represented by (.+-.)] compounds.
That is, compounds represented by the formula (III) ##STR2## wherein R.sub.3 has the same significance as defined above, R.sub.2 represents a hydrogen or a lower alkyl group and the hydrogen atoms at the 6- and 7-positions have cis configuration are present as a mixture of equal amounts of the mirror image compounds represented by the formulae (III-1) and (III-2) ##STR3##
As a result, the acyl compounds derived from such carbacephem compounds are also optically inactive.
On the other hand, it is known that an optically active acyl group, such as a D-phenylglycyl group, can be introduced to an optically inactive carbacephem compound, for example, a compound represented by the formula (IV) ##STR4## wherein R.sub.2 ' represents a hydrogen or a methyl group and thereafter separated to diastereoisomers as is described in a Reference Example hereinafter and Japanese Patent Application No. 127027/78. However, this method is complicated because in the preparation of Compound (IV) protection and elimination processes of the amino group or carboxyl group are required.
In Japanese Patent Application No. 14533/79 and U.S. patent application Ser. No. 119451, filed Feb. 7, 1980, of the present inventors, optically active carbacephem compounds are prepared by microbial enzymatic action. This method is, however, also complicated in that it requires a reaction to introduce an acyl group to an optically active carbacephem compound obtained separately in order to obtain an acyl derivative having an optically active carbacephem ring.
There have been reports of the enzymatic acylation of naturally occurring .beta.-lactam compounds such as 6-aminopenicillanic acid which is the ring of penicillins, 7-aminocephalosporanic acid and 7-aminodeacetoxycephalosporanic acid which are the ring of cephalosporins. However, there have been no reports of successful enzymatic acylation of a .beta.-lactam compound synthetically prepared having a carbacephem ring.
Therefore, a need exists for a direct process by which optically active synthetic carbacephems can be prepared. To this end, it has now been found that optically active acyl derivatives having a carbacephem ring can be directly prepared by an enzymatic reaction using microorganisms. The fact that such microorganisms have an ability for acylating with absolute optical selectivity a carbacephem compound synthetically prepared is a novel finding.